1. Field of the Invention
The present invention relates to a digital transmission equipment and in particular to an equipment which makes a digital signal transmission through a wireless communication link (radio communication link) in a digital transmission system.
2. Description of the Related Art
In such a digital transmission equipment, the information of voices, images, and data, etc. is digitized and multiplexed to a higher-order group digital signal which is transmitted to a communication link. This kind of equipment may use a method of transmission with a high-speed digital communication link through an access line (link) or with a wireless communication link of microwave or millimeter wave.
FIG. 9A shows a digital transmission equipment using the former method of transmission with a high-speed digital line.
As shown in the figure, this digital transmission equipment is composed of a multiplexer 40A and an optical access line digital service unit (abbreviated as DSU) 41A. The multiplexer 40A transmits a multiplexed digital signal of 6 Mbps to the digital service unit 4A which is supplied with a reference clock (not shown) for synchronization, and communicates with an optical access line digital service unit 41B and a multiplexer 40B in a digital transmission equipment as opposed through a transmission wired line (cable line) L6 which is a high-speed digital leased line.
Being generally used, this digital transmission equipment malfunctions when a failure occurs on the transmission wired line L6 for instance. Also, if a public or external access line is employed as the transmission line between near buildings or in a factory, an extra cost of communication expense will be required.
Then, a digital transmission equipment using the latter method of transmission with a wireless communication link L3 as shown in FIG. 9B has now been developed.
This digital transmission equipment is composed of a multiplexer 40A (40B), an access line digital service unit 42A (42B), and a transmitter-receiver (abbreviated as MRU) 42A (42B). The multiplexers 40A and 40B are connected to the access line digital service units 41A and 41B, through the transmission lines L1 and L5, respectively, which are connected to the transmitter-receivers 42A and 42B through transmission lines L2 and L4, respectively. The transmitter-receivers 42A and 42B are mutually connected through a wireless communication link L3.
Thus, the multiplexers 40A-40B are mutually connected by two parallel links between the transmitter-receivers 43A and 43B and the access line digital service units 41A and 41B whereby the failure thereof can be backed up and a private network of a large capacity can be constructed without using any other public access line.
In such a digital transmission equipment, a signal frame format used on the wireless communication link L3 is shown in FIG. 10. This frame is composed of frame synchronization bits (channel) F.sub.SR (3 octets), route (line) identification bits ID (4 bits) which specify a radio connection between opposed transmitter-receivers, an own terminal (DTE) state indication bit UAIS (or a state indication bit S) (1 bit), free bits (3 bits), loopback testing bits (3 bits) which contain bits LPBA-LPBC, data transfer bits TS.sub.1 -TS.sub.98 (H channel+D channel+free channel) (98 octets), and CRC-5 check bits e.sub.k1 -e.sub.k5 (5 bits) for the wireless signal frame. The speed is 6.592 Mbps.
In this wireless signal frame, the above-noted bits ID, UAIS/S and LPBA-LPBC can be referred to as "service information bits" which are transferred between transmission equipments for the purpose of supervision/maintenance. The synchronization bits F.sub.SR form an overhead of the wireless signal frame together with the service information bits ID, UAIS/S and LPBA-LPBC. The bits TS.sub.1 -TS.sub.98 and e.sub.k1 -e.sub.k5 are data bits which are extracted from the output signal of the multiplexers 40A and 40B and are mapped into the wireless signal frame which is to be transmitted via the wireless communication link L3.
Accordingly, in such a digital transmission equipment, unless a format conversion where a wireless signal frame is newly generated to add some bits is done, the bits ID, UAIS/S and LPBA-LPBC which are peculiar to the wireless communication link cannot be transferred. Therefore, there arises a problem of circuit arrangements being complicated and speed conversion circuits being required.
Thus, an equipment for digital signal transmission without newly adding a bit peculiar to the wireless communication link has been required.
As to the above-mentioned digital transmission equipment without requiring new addition of a bit peculiar to wireless communication link, the inventor of this invention (with a joint inventor) has already proposed same in the Japanese Patent Laid-open Publication No. 1-293031, as shown in a digital transmission system in FIG. 11.
In this prior art, the digital transmission system has a wireless link section formed by the division of a wireless terminal station as a digital transmission equipment into a master and a slave station provided with a reference clock signal (not shown) by the master station. The wireless terminal station in the master station includes a multiplexer 50A, an interface apparatus 51A which normally serves as an interface unit and is connected to the multiplexer 50A through the transmission line L1, and a transmitter receiver (MRU) 52A connected to the interface unit 51A through the transmission line L2. The wireless terminal station in the slave station connected via the wireless communication link L3 to the master station similarly includes a multiplexer 50B, an interface unit 51B connected to the multiplexer 50B through the transmission line L5, and a transmitter-receiver 52B connected to the interface unit 51A through the transmission line L4.
Also, in the wireless link section, the multiplexers 50A and 50B are connected through the optical access line digital service units 53A and 53B as well as the high speed leased line L6 (see FIG. 9A) which can be a protecting line for the wireless communication link L3 as a working link
In addition the multiplexer 50A is connected to a data processing device 54, a telephone or facsimile receiver 56, and a television conference device 57 which form data terminal equipments, respectively. The telephone or facsimile receiver 56 is connected to the multiplexer 50A through PBX 55 (Private Branch exchange). The same arrangement applies to the multiplexer 50B, although not being shown in FIG. 11.
In this arrangement, the interface units 51A and 51B are substituted for the access line digital service units 42A and 42B in FIG. 9B, respectively. The interface units 51A and 51B interface the transmitter-receivers 52A, 52B with the multiplexers 50A, 50B, respectively when the opposed digital transmission equipments communicate with each other across the wireless link section.
Namely, an output signal from the data terminal equipments 54-57 is multiplexed e.g. by the multiplexer 50A of the master station, and is converted into a frame format called Y-interface specification which is the local (Japanese) standard as shown in FIG. 12.
This frame format has data transfer bits TS.sub.1 -TS.sub.98 (98 bytes) and service information bit SF as shown in FIG. 12A. Similarly to the frame format in FIG. 10, for the purpose of supervision/maintenance between the transmission equipments, the service information bit SF includes 5 bits, in each of 4 continuous frames composing a superframe (multi-frame of 20 bits), which correspond to the CRC-5 check bits e.sub.k1 -e.sub.k5 among the wireless signal frame shown in FIG. 10. The CRC-5 check bits e.sub.k1 -e.sub.k5 in the form of the superframe include a bit CRV (frame synchronization bit) in the first frame and a bit S (state indication bit) in the third frame, respectively at a predetermined position, as shown in FIG. 12B. Other bits shown by "--" are not assigned for particular bits according to the Y-interface specification.
By the signal reception of this frame, the interface unit 51A produces a wireless signal frame.
In this wireless signal frame, the data transfer bits TS.sub.1 -TS.sub.98 are not changed as shown in FIG. 12C. As for the service information bit SF, parity bits P are inserted in the tail of every frame, and route identification bits ID (4 bits) are inserted in the second (or fourth) frame of the superframe, for the quality supervision of the transmission line.
Besides, a signal F (FFF=111) of 3 bits is inserted to make a frame synchronization and the position where the bit CRV is inserted is changed for parity bits P. Also, a bit UNR which shows a failure occurrence up to this time point is inserted in a fixed position.
Synchronization for the wireless communication link is achieved for a frame of the Y-interface specification by allocating the above-mentioned service information bit SF into a superframe.
This frame is restored to the original frame format in the interface unit 51B after being transmitted to the transmitter-receiver 52B of the slave station opposed through the transmitter-receiver 52A and the wireless communication link L3, is separated into the data terminal equipment (not shown) through the multiplexer 50B and is transmitted. It is similar for the opposite direction from the slave to the master station.
The interface unit observes the service information bit SF in the frame having been transferred and measures the code error rate. When the code error rate exceeds a predetermined value, the interface unit generates an alarm signal. Also, by generating a test pattern during a test time the interface unit transmits the alarm signal based on the test results.
If the wireless signal frame according to the aforementioned Y-interface specification shown in FIG. 12B is realized with the I-interface specification which is now internationally demanded, it will be a frame composition shown in FIG. 13.
Specifically, four bits in the first frame in the superframe are allocated to synchronization bits F.sub.s in the wired signal frame, and one bit is allocated to a UNI (User Network Interface) maintenance signal bit m. In the second frame, all of the five bits are allocated to the synchronization bits F.sub.s.
In the third frame, one bit is allocated to the state indication bit S between interface units, three bits to the loopback testing bits LPBA-LPBC, and one bit to the maintenance bit m, respectively. In the fourth last frame, the original CRC-5 check bits e.sub.k1 -e.sub.k5 are allocated without any change.
Namely, in this wireless signal frame, the synchronization bits F.sub.s remain unchanged while other service information bits S and LPBA-LPBC are added. Recalculated CRC-5 check bits e.sub.k1 -e.sub.k5 are substituted for the original check bits e.sub.1 -e.sub.5 upon CRC operation incorporating the newly added service information bits mentioned above.
The bit positions for the bits S, LPBA-LPBC and m in the third frame are currently allocated as reserved bits X and a nonuse bit a (shown in the frame formats (A) and (D) of FIG. 1A) in the frame format of the I-interface specification for UNI (wired line).
For more effective state indication and maintenance of the wireless communication link, more service information bits such as line identification bits could advantageously be utilized.
However, no more reserved bits or nonuse bits are available in the I-interface specification. In addition, arbitrary use of the currently reserved or nonuse bits might cause inconvenience in the future because they may possibly be used for other purposes.